The invention relates to an electric machine having a potential equalization device for breaking down shaft voltages, the electric machine having a stator and a rotor mounted rotatably by means of a rotor shaft and having a potential equalization device containing a resistor element for breaking down shaft voltages occurring at the rotor shaft, the resistor element contacting on the one hand a bus bar of a slip ring module and on the other hand the rotor shaft at contact points.
Electric machines are being used increasingly in motor vehicles as drive machines that also serve in overrun conditions as generators for energy recovery. Electric machines of this type are known by the name “boost-recuperation machines”.
It is known that shaft voltages may form on the rotor shaft during operation in the case of electric machines and may constitute a voltage difference to the machine ground and may have different causes. One cause for this may be, for example, the clocked actuation of the stator or rotor winding by means of a pulse-width-modulated signal. In the case of this cause higher shaft voltages occur, the higher is the supply voltage of the on-board power system and therefore the operating voltage of the electric machine.
A further possible cause, however is also that electrostatic charges lead to high shaft voltages. By way of example, an electric machine operated as generator may be driven via a belt, and therefore the rotor may be electrically charged via the belt.
The occurring shaft voltages discharge in a pulse-like manner via a bearing, preferably via the drive-side ball bearing, and this therefore results in a pulse-shaped current flow through the bearing. Here, spark erosion may occur in the region of the bearing surfaces, which may lead to damage of the bearing. Spontaneous discharges of this type via a bearing of the rotor may occur in particular in the event of a failure of the insulation effect of the bearing grease in the form of a pulse-shaped current flow, whereby a permanent loading of the bearing damages said bearing and the service life of the bearing is considerably reduced.
Furthermore, the pulse-like discharge of the charge carriers located on the rotor shaft leads to the creation of electromagnetic waves, which negatively influence the electromagnetic compatibility of the product. In particular, this may hinder the fault-free function of electrical devices located in the vicinity.
An arrangement for dissipating shaft voltages for electric machines is known from DE 35 11 755 A1, in which charge carriers are diverted from the rotor shaft via a slip contact element. For this purpose a capacitor effective between the rotor shaft and machine ground is provided and may be connected in series with an inductor. The arrangement for dissipating shaft voltages may also be referred to as a potential equalization device, since a potential equalization is attained with this between ground or for example between the negative pole and the rotor shaft.
An alternating current generator is known from DE 101 18 004 A1, which comprises a stator and a rotor mounted rotatably by means of a rotor shaft. In this known electric machine an electrically conductive coating is applied to the rotor shaft and enables electrostatic charges from the rotor shaft and a slip rig of a slip ring group to drain away.